Acquired performance is not merely a direct reflection of associative strength of the eliciting stimulus. Our recent studies of Pavlovian conditioning with rats indicate that responding depends on the associative strength of the CS relative to the associative strength of other cues, including context, present during training (not testing). This comparison apparently occurs at the time of testing (not training). We have demonstrated comparator process contributions to response generation on tests of conditioned inhibition (CI), conditioned excitation, US preexposure, and overshadowing. This comparator process, in combination with other established phenomena, makes traditional concepts of CI superfluous. Rather, an effective conditioned inhibitor can be viewed merely as a cue that is less excitatory than its comparator stimulus, with responding to the putative inhibitor reflecting a decrease in the likelihood of the US relative to background. This view appears to be valid for CI produced through either negative contingency training or Pavlov's A+/AX- procedure. We have completed analyzes of "retardation" and "superconditioning" following CI training, without recourse to any of the traditional concepts of CI. An analysis of summation with conditioned inhibitors is in progress and appears to be forcing us towards a modified version of Pearce and Hall's proposal that there are two independent types of attention, one for acquisition and another for performance. Our primary goal is to illuminate the comparator process and its constraints, e.g., how close to a CS in time and space must an event be for it to be compared to the CS. Additional research seeks to understand the comparator process contribution to operational extinction of CI, learned irrelevance, and blocking. Further studies will determine under what conditions an excitatory training context that is present during testing will attenuate (by comparator processes) as opposed to augment (by associative summation) conditioned responding. We also want to determine if comparator stimuli have comparator stimuli of their own, and if so do such stimuli modulate responding to CSs as well as direct responding to the first level comparator stimuli.